CMR-10128: Adding metadata xml file back to assets

[william-valencia]

CMR-STAC users were complaining that the metadata xml file is missing from the assets field in CMR-STAC. Adding this back for them.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.88%. Comparing base (55915b0) to head (a9a9eed). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #354 +/- ## ========================================== + Coverage 87.83% 87.88% +0.05% ========================================== Files 23 23 Lines 1118 1123 +5 Branches 241 241 ========================================== + Hits 982 987 +5 Misses 136 136 ```

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[william-valencia]